1. Field of the Invention
This invention relates to solid electrolyte fuel cells, and more particularly provides a generator system comprised of such cells.
2. Description of the Prior Art
High temperature solid electrolyte fuel cells convert chemical energy into direct current electrical energy, typically at temperatures above 700.degree. C. This temperature range is required to render the solid electrolyte sufficiently conductive for low power losses due to ohmic heating. With such cells, expensive electrode catalysts and refined fuels are not required. For example, carbon monoxide-hydrogen fuel mixtures can be used directly, without conversion. Stabilized zirconia is a prime electrolyte candidate, and is used in thin layers on ceramic tubular support structures. The support tubes for thin film high temperature solid oxide electrolyte cells are generally also made of stabilized zirconia and serve as ducts for one of the reactants, fuel or oxidant. This requires porosity in the support tubes.
Many such fuel cells must be connected electrically in series for high voltages, since each cell has a terminal voltage of approximately one volt. A problem arises in the construction of larger generators because the fuel and oxidant, such as air, must be preheated to temperature that require high temperature heat exchangers, such as those comprised of ceramics, a technology that is, for present purposes, economically unavailable.
Large ceramic assemblies for high temperature operation, such as furnaces, usually consist of small building blocks that allow for free thermal expansion and thus alleviate cracking which otherwise could destroy such structures in a uncontrollable manner. It has, therefore, been assumed that high temperature solid oxide electrolyte fuel cells must have construction features similar to existing larger ceramic structures used at high temperatures.
Sealing of such fuel cells has been a related concern because reactants must be separated. This is a result not only of the need to separate the fuel and oxidant to avoid interaction other than electrochemical combustion, but also to avoid harming the electrodes which typically can operate desirably only in either a fuel, or an oxidant, environment.
In such systems, fuel consumption is not complete and five to fifteen percent will remain in the anode exhaust. Similarly, an oxidant, such as air, which typically also functions as a coolant, is depleted in the fuel cells, although the oxygen depletion of air is low. The depleted fuel is not utilized to its full capacity. To date, no economically and technically feasible systems have been proposed for the construction of high temperature solid oxide electrolyte fuel cell generators. Most concepts propose shell and tube heat exchanger type structures which rigidly seal fuel cell tubes either with ceramic or metal-to-ceramic seals. Such seals are complex, and have raised concerns regarding reliability.
It is desirable to provide high temperature solid oxide electrolyte generating systems which alleviate these and other concerns and which provide reliable, efficient means of generating useful energy.